Applying epitaxial silicon in disposable spacer flow

ABSTRACT

A process for forming active transistors for a semiconductor memory device by the steps of: forming transistor gates having generally vertical sidewalls in a memory array section and in periphery section; implanting a first type of conductive dopants into exposed silicon defined as active area regions of the transistor gates; forming temporary oxide spacers on the generally vertical sidewalls of the transistor gates; after the step of forming temporary spacers, implanting a second type of conductive dopants into the exposed silicon regions to form source/drain regions of the active transistors; after the step of implanting a second type of conductive dopants, growing an epitaxial silicon over exposed silicon regions; removing the temporary oxide spacers; and forming permanent nitride spacers on the generally vertical sidewalls of the transistor gates.

This application is a continuation to U.S. patent application Ser. No.11/305,597, filed Dec. 16, 2005, which is a continuation of U.S. patentapplication Ser. No. 10/861,438, filed Jun. 3, 2004, which is adivisional to U.S. Pat. No. 6,448,129 B1, filed Jan. 24, 2000.

FIELD OF THE INVENTION

This invention relates to semiconductor fabrication processing and, moreparticularly, to a method for forming active devices for semiconductorstructures, such as field effect transistors used in random accessmemories.

BACKGROUND OF THE INVENTION

Conventional fabrication techniques used to form the active transistorsin memory devices have led to several undesirable results. It has becomecommon practice to form active transistors with spacers on the verticalwalls of the transistor gates by first forming disposable spacers andhaving the oxide spacers in place during conductive doping implantationsteps to form the source/drain regions of the transistors. Thedisposable oxide spacers are eventually removed and replaced with finalspacers that possess a desired spacer thickness.

However, during the final spacer etch, when nitride is used as thespacer material, it is difficult to etch the nitride spacer with highselectivity to silicon and oxide and yet insure that all of the nitrideis cleared from the source/drain regions of the active transistors.Because of this difficulty, a portion of the field oxide may be removedalong with a portion of the silicon substrate that has been implantedwith conductive dopants to form the transistor's source/drain regions.Etching into the field oxide can lead to transistor junction currentleakage, while etching into the silicon source/drain region can lead tohigh source/drain resistance or even open circuits. If either of theseconditions occurs, they will adversely affect transistor operation.

The present invention discloses a method to form active transistors in asemiconductor memory device that will protect the source/drain region ofthe active transistors during a spacer etch sequence so as tosubstantially reduce high source/drain resistance and leakage that mayoccur in the transistor junction.

SUMMARY OF THE INVENTION

Exemplary implementations of the present invention comprise processesfor forming active transistors for a semiconductor memory device.

A first exemplary implementation of the present invention utilizes theprocess steps of forming transistor gates having generally verticalsidewalls in a memory array section and in periphery sections.Conductive dopants are implanted into exposed silicon defined as activearea regions of the transistor gates. Disposable (temporary) spacers areformed on the generally vertical sidewalls of the transistor gates.Epitaxial silicon is grown over exposed silicon regions. After theepitaxial silicon is grown, conductive dopants are implanted into theexposed silicon regions to form source/drain regions of the activetransistors. The temporary spacers are removed and permanent insulativespacers are formed on the generally vertical sidewalls of the transistorgates.

A second exemplary implementation of the present invention utilizes theprocess steps listed above, but more specifically, the temporary spacersare formed of oxide and the permanent spacers are formed of nitride.

A third exemplary implementation of the present invention utilizes theprocess steps of the first exemplary implementation except that thesource/drain regions of the active transistor are formed prior to theformation of the epitaxial silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view depicting a semiconductor substrateafter the formation of active transistors in the array and periphery ofa semiconductor memory device, including a Light Drain Doping (LDD)implant and a source/drain doping implant of both n-channel andp-channel transistors.

FIG. 1B is a cross-sectional view of the structure of FIG. 1A takenafter the removal of the temporary oxide spacers and the growth ofepitaxial silicon or epitaxial silicon germanium at the exposeddiffusion regions of the active transistors.

FIG. 1C is a cross-sectional view of the structure of FIG. 1B takenafter the formation of silicon nitride spacers along the substantiallyvertical walls of each transistor gate.

FIG. 2A is a cross-sectional view depicting a semiconductor substrateafter the formation of active transistors in the array and periphery ofa semiconductor memory device, including a Light Drain Doping (LDD)implant of both n-channel and p-channel transistors.

FIG. 2B is a cross-sectional view of the structure of FIG. 2A takenafter the growth of epitaxial silicon or epitaxial silicon germanium atthe exposed diffusion regions of the active transistors, followed by asource/drain doping implant of both n-channel and p-channel transistors.

FIG. 2C is a cross-sectional view of the structure of FIG. 2B takenafter the removal of temporary oxide spacers and the formation ofpermanent silicon nitride spacers along the substantially vertical wallsof each transistor gate.

FIG. 3A is a cross-sectional view depicting a semiconductor substrateafter the formation of active transistors in the array and periphery ofa semiconductor memory device, including a Light Drain Doping (LDD)implant of both n-channel and p-channel transistors.

FIG. 3B is a cross-sectional view of the structure of FIG. 3A takenafter the growth of epitaxial silicon or epitaxial silicon germanium atthe exposed diffusion regions of the active transistors, followed by asource/drain doping implant of both n-channel and p-channel transistors.

FIG. 3C is a cross-sectional view of the structure of FIG. 3B takenafter the removal of temporary oxide spacers and the formation ofpermanent silicon nitride spacers along the substantially vertical wallsof each transistor gate.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary implementations of the present invention directed to processesfor forming active transistors, in a semiconductor device, are depictedin FIGS. 1A-3C.

A first exemplary implementation of the present invention is depicted inFIGS. 1A-1C. In the drawings of FIGS. 1A-1C, the semiconductor assemblyrepresents a memory device partitioned into three main sections: memoryarray section 10A, n-channel periphery section 10B and p-channelperiphery section 10C. FIG. 1A depicts a semiconductor assembly 11, suchas a silicon wafer, that has been processed to a particular point.

Referring to FIG. 1A, processing steps comprising transistor gate stackdeposition, followed by patterning and etching of the gate stack areused to form transistor gates 13A in memory array section 10A,transistor gates 13B in n-channel periphery section 10B, and transistorgates 13C in p-channel periphery section 10C. Following the formation ofthe transistor gates, a lightly doped drain (LDD) phosphorus implant isperformed to form lightly doped p-type regions 14A, 14B, and 14C intosilicon substrate 11, except where field oxide 12 is present. Followingthe LDD phosphorus implant, a nitride layer is deposited over transistorgates 13A, 13B and 13C, over exposed portions of silicon substrate 11and over field oxide 12. After the deposition of the nitride layer,oxide having a thickness that is greater than one half the width of thespacing between transistor gates 13A (in the memory section 10A), isdeposited over the nitride layer. An oxide spacer etch is performed toform temporary oxide spacers 16A, 16B, and 16C. Due to the thickness ofthe oxide layer and to the length of the oxide etch, oxide spacers 16Aseal off the underlying silicon between memory transistor gates 13A. Theoxide etch will also clear the nitride underlying the oxide and stoponce silicon substrate 11 is reached. The oxide etch will also removethe thin nitride layer from the surface of the transistor gates 13B.Since the transistor gate stack typically includes nitride as the topmaterial, each transistor gate remains coated with nitride. The oxideetch also forms nitride liners 15A, 15B and 15C, in memory section 10A,periphery n-channel section 10B, and in periphery p-channel section 10G,respectively. Memory section 10A and n-channel periphery section 10B aremasked off and a p-channel source/drain implant is performed to formp-channel source/drain regions 17C. The mask over memory section 10A andn-channel periphery section 10B is stripped and p-channel peripherysection 10C is then masked off. Next, an n-channel source/drain implantis performed to form source/drain regions 17B.

Referring now to FIG. 1B, after the n-channel source/drain implant, themask over p-channel periphery section 10C is stripped and an oxide wetetch is performed to remove temporary oxide spacers 16A, 16B, and 16Cshown in FIG. 1A. Using the exposed portions of silicon substrate 11, atsource/drain regions 17B and 17C, an epitaxial silicon or silicongermanium material is grown to form epitaxial extension regions 18B and18C. At epitaxial extension regions 18B and 18C, the epitaxial materialwill not only grow upward, but outward as well, resulting in a portionof epitaxial material to grow beyond the boundaries of the exposedsilicon surface.

Referring now to FIG. 1C, a nitride layer is deposited over thesemiconductor assembly using conventional nitride deposition techniques.Next, a nitride spacer etch is performed to form permanent nitridespacers 19A, 19B and 19C. During the nitride spacer etch, epitaxialmaterial 18B and 18C is reduced according to the length of time thenitride etch is performed. During the nitride spacer etch, it iscritical that the nitride is removed from the surface of the epitaxialmaterial and yet sufficient nitride material remains to seal off thesurface of silicon substrate 11 at the base of nitride spacers 19A, 19Band 19C.

For example, if the space between transistor gates 13A, in memory arraysection 10 a, is 0.2 microns, a 0.12 microns thick temporary spacer(16A) can be deposited to fill the space. A wet etch can be used toremove 0.04 microns of the temporary spacer so that the final spacerthickness of temporary spacers 16B and 16C is 0.08 microns. After thesource/drain implant and a second wet etch is performed to remove thedisposable oxide (spacers 16A, 16B and 16C), an epitaxial material (18Band 18C) is grown to a thickness of 0.06 microns. Following epitaxialmaterial growth, the nitride layer used to form nitride spacers 19A, 19Band 19C is deposited to a thickness of 0.04 microns. Next, by using anonselective etch, the epitaxial material and the nitride material areremoved at the same rate. To ensure the 0.04 micron thick nitrideoverlying the epitaxial material is completely removed, a 50% over etchis used that will not only remove the 0.04 microns nitride layer, butalso remove 0.02 microns of the epitaxial material. This etch willresult in the formation of nitride spacer thickness of 0.04 microns andan epitaxial material thickness of 0.04 microns, which guarantees thethat the silicon surface of source/drain diffusion regions 17B and 17Cwill not be etched.

Thus, the epitaxial material protecting the source/drain diffusionregions 17B and 17C from the above mentioned nitride spacer etch helpsto prevent high source/drain resistance that can occur when a portion ofthe silicon containing the source/drain region is removed. The presenceof the epitaxial material also allows for the formation of shallowtransistor junctions. Another function of the epitaxial material is toprotect the field oxide 12 during the nitride spacer etch to preventtransistor junction leakage (current) that can result. The process flowof the present invention improves transistor isolation and provides aprocess flow that that is highly scalable for geometrically smallerdevices. The semiconductor device is then completed in accordance withfabrication processes known to those skilled in the art.

A second exemplary implementation of the present invention is depictedin FIGS. 2A-2C. In the drawings of FIGS. 2A-2C, the semiconductorassembly represents a memory device partitioned into three mainsections: memory array section 20A, n-channel periphery section 20B andp-channel periphery section 20C. As in the first exemplaryimplementation, FIG. 2A depicts a semiconductor assembly 21, such as asilicon wafer, that has been processed to a particular point.

Referring to FIG. 2A, processing steps described in the first exemplaryimplementation are used to form transistor gates 23A in memory arraysection 20A, transistor gates 23B in n-channel periphery section 20B,and transistor gates 23C in p-channel periphery section 20C. Followingthe formation of the transistor gates, a lightly doped drain (LDD)phosphorus implant is performed to form lightly doped p-type regions24A, 24B, and 24C into silicon substrate 11, except where field oxide 22is present. Following the LDD phosphorus implant, a nitride layer isdeposited over transistor gates 23A, 23B and 23C, over exposed portionsof silicon substrate 21 and over field oxide 22. After deposition of thenitride layer, oxide having a thickness that is greater than one halfthe width of the spacing between transistor gates 23A (in the memorysection 20A), is deposited over the nitride layer. An oxide spacer etchis performed to form temporary oxide spacers 26A, 26B, and 26C. Due tothe thickness of the oxide layer and to the length of the oxide etch,oxide spacers 26A seal off the underlying silicon between memorytransistor gates 23A. The oxide etch will also clear the nitrideunderlying the oxide and stop once silicon substrate 21 is reached. Theoxide etch also forms nitride liners 25A, 25B and 25C, in memory section20A, periphery n-channel section 20B, and in periphery p-channel section20C.

Referring now to FIG. 2B, using the exposed portions of siliconsubstrate 21, an epitaxial silicon or silicon germanium material isgrown to form epitaxial extension regions 27B and 27C. At epitaxialextension regions 27B and 27C, the epitaxial material will not only growupward, but outward as well, resulting in a portion of epitaxialmaterial to grow beyond the boundaries of the exposed silicon surface.Memory section 20A and n-channel periphery section 20B are masked offand a p-channel source/drain implant is performed to form p-channelsource/drain regions 28C, which include epitaxial extension regions 27C.The mask over memory section 20A and n-channel periphery section 20B isstripped and p-channel periphery section 20C is then masked off. Next,an n-channel source/drain implant is performed to form source/drainregions 28B, which includes epitaxial extension regions 27B. Thepresence of epitaxial extension regions 27B and 27C, become important asis discussed later in the process.

Referring to FIG. 2C, after the n-channel source/drain implant, the maskover p-channel periphery section 20C is stripped and an oxide wet etchis performed to remove temporary oxide spacers 26A, 26B, and 26C, shownin FIG. 2B. Following the oxide etch, a nitride layer is deposited overthe semiconductor assembly using conventional nitride depositiontechniques and a nitride spacer etch is performed to form permanentnitride spacers 29A, 29B and 29C. During the nitride spacer etch,epitaxial material 27B and 27C is reduced according to the length oftime the nitride etch is performed. The ideal etch is timed so that theentire nitride material is removed from the surface of the epitaxialmaterial and yet none of the silicon from the silicon surface (i.e.,silicon substrate 21) of source/drain diffusion regions 28B and 28C isremoved. The epitaxial material protecting the source/drain diffusionregions 28B and 28C from the above mentioned nitride spacer etch helpsto prevent high source/drain resistance that can occur when a portion ofthe silicon containing the source/drain region is removed. The presenceof the epitaxial material also allows for the formation of shallowtransistor junctions. Another function of the epitaxial material is toprotect the field oxide 22 during the nitride spacer etch to preventtransistor junction leakage (current) that can result. The process flowof the present invention improves transistor isolation and provides aprocess flow that that is highly scalable for geometrically smallerdevices. The semiconductor device is then completed in accordance withfabrication processes known to those skilled in the art.

A third exemplary implementation of the present invention is depicted inFIGS. 3A-3C. In the drawings of FIGS. 3A-3C, the semiconductor assemblyrepresents a memory device partitioned into three main sections: memoryarray section 30A, n-channel periphery section 30B and p-channelperiphery section 30C. As in the second exemplary implementation, FIG.3A depicts a semiconductor assembly 31, such as a silicon wafer, thathas been processed to a particular point. The processing steps of thesecond exemplary implementation are used to develop memory section 30A,n-channel periphery section 30B and p-channel periphery section 30C.However, there is a variation in the development of these sections andin particular with memory section 30A, which is described later. Asshown in FIGS. 3A-3C, field oxide 32 is formed in silicon substrate 31,transistor gates 33A, 33B and 33C, vertically surrounded by oxidespacers 36A, 36B and 36C, are formed on top of silicon substrate 31, anddiffusion regions, including lightly doped regions 34A, 34B and 34C,epitaxial extension regions 37A, 37B and 37C and source/drain regions38A, 38B and 38C, are formed as well.

As mentioned above, a variation in the process changes the resultingsemiconductor assembly. Instead of depositing oxide to the thicknessdescribed in the second exemplary implementation of the presentinvention, the oxide layer used to form oxide spacers 36A, 36B and 36Cis deposited to a thickness that is less than one half the width of thespacing between transistor gates 33A (in the memory section 30A). Oncethe oxide spacer etch is performed, oxide spacers 36A will have a gapbetween them that will allow the formation of epitaxial region 37A,shown in FIG. 3B.

Referring to FIG. 3C, a nitride layer is deposited over thesemiconductor assembly using conventional nitride deposition techniques.A nitride spacer etch is performed to form permanent nitride spacers39A, 39B and 39C, which are present on the vertical edges of epitaxialgrowth 38A, 38B and 38C, as well as on the vertical walls of transistorgates 33A, 33B and 33C. During the nitride spacer etch, epitaxialmaterial 37A, 37B and 37C is reduced according to the length of time thenitride etch is performed. The ideal etch is timed so that nitridematerial is removed from the surface of the entire epitaxial materialand yet none of the silicon from the silicon surface (i.e., siliconsubstrate 31) of source/drain diffusion regions 38A, 38B and 38C isremoved. The semiconductor device is then completed in accordance withfabrication processes known to those skilled in the art.

In the above exemplary implementations of the present invention, desiredconditions to form an epitaxial silicon material comprise presenting agas flow of 5 slm of hydrogen (H₂), 50 sccm of dichlorosilane (DCS) and15 sccm of hydrochloric acid (HCl) to the surface of silicon assembly11, with the processing chamber temperature at 825° C. and the pressureset at 133 Pa. Desired conditions to form an epitaxial silicon germaniummaterial comprise presenting a gas flow of 5 slm of H₂, 100 sccm of DCS,20 sccm of GeH₄ (10% H₂) and 20 sccm of HCl to the surface of siliconassembly 11, with the processing chamber temperature at 750° C. and thepressure set at 400 Pa.

It is to be understood that although the present invention has beendescribed with reference to several preferred embodiments, variousmodifications, known to those skilled in the art, may be made to theprocess steps presented herein without departing from the invention asrecited in the several claims appended hereto.

1. A method of fabricating transistors on a semiconductor substratecomprising: forming transistor gates of first and second transistorslocated in first and second areas of the semiconductor substrate,respectively, wherein the transistor gates have generally verticalsidewalls; simultaneously forming source and drain regions for the firstand second transistors; forming temporary spacers on the verticalsidewalls of the first and second transistor gates, wherein thetemporary spacers of the first transistor abut a semiconductor structuresuch that the source and drain regions of the first transistor arevertically covered and the temporary spacers of the second transistorcover a portion of the source and drain regions of the second transistorsuch that a portion of the source and drain regions remain exposed; andexposing the semiconductor substrate to an implant dopant to change thedopant level of the exposed portions of the source and drain regions ofthe second transistors.
 2. The method of claim 1 further comprisesforming epitaxial regions above the exposed portions of the source anddrain regions of the second transistors.
 3. A semiconductor devicecomprising: first and second transistors located in first and secondareas of the semiconductor substrate, the first and second transistorseach having source and drain regions, wherein the source and drainregions of the first transistors have a single dopant level and thesource and drain regions of the second transistor comprises multipledopant levels; and epitaxial material above portions of the source anddrain regions of the second transistors.
 4. A semiconductor device,comprising: first and second transistors located in first and secondareas of a semiconductor substrate, respectively; the first transistorcomprising a source and drain region with a first level of doping; thesecond transistor comprising a source and drain region with a secondlevel of doping; and epitaxial silicon above at least a portion of thesource and drain region of the second transistor.
 5. The device of claim4, wherein the second level of doping is deeper than the first level ofdoping.
 6. A semiconductor device comprising: a first and secondplurality of transistors located in first and second areas of thesemiconductor substrate, the first and second plurality of transistorseach having source and drain regions, wherein the source and drainregions of the first plurality of transistors have a single dopant leveland the source and drain regions of the second plurality of transistorcomprises multiple dopant levels; and spacers adjacent to generallyvertical side walls of gates of the first and second plurality oftransistors, wherein a lateral distance between the gates of the firstplurality of transistors is less than twice a width of the spacers. 7.The device of claim 6 further comprising epitaxial material aboveportions of the source and drain regions of the second plurality oftransistors.